Universal flight indicator



Nov. 3, 1 942.

E. A. SPERRY, JR

UNIVERSAL FLIGHT INDICATOR 4 Shefs-Sheet 1 Filed May 4, 1937 INVENTOR Elmer 14. 5

3, 9 E. A. SPERRY, JR

UNIVERSAL FLIGHT INDICATOR 4 Sheets-Sheet 5 Filed May 4, 1957 v mmvrox Elmerfl. Spgrgji ATTRNEY I Nov. 3, 1942.

E. A. SPERRY, JR

UNIVERSAL FLIGHT INDICATOR I Filed May 4, 1937 4 Sheets-Sheet; 4

'l /lll .4

IN VEN TOR ,flmer ,4. Sperr ,Jr.

ATTORNEY the magnet.

excess of 180.

Patented Nov. 53;, 1942 i UNIVERSAL FLIGHT INDICATOR Elmer A. Sperry, Jr., Brooklyn, N.

Y., or to Sperry Products, Inc, Brooklyn, N. Y., acorporation of New York Substituted for abandoned applications Serial No. 756,425, December 7, 1934, and Serial No.

46,633, October 25, 1935. ITEMS 4, 1937, Serial No. 140,650

application May 7 Claims. (oi. 33-223) herent characteristics, performs the functions" not only of a magnetic compass but also of an artificial horizon, directional gyro, and inclinometer. Y

This application is a continuation of my copending applications Serial Nos. 756,425, filed December 7, 1934, and 46,638, filed October 25, 1935, both now abandoned.

It is well known that the standard magnetic compass cannot be used by the pilot of an airplane for the purpose of flying a straight course or making turns, without the aid of other instruments or visual ground references. This is due primarily to two factors, first, the magnetic dip of the magnets is overcome in the standard magnetic compasses by counterweighting one end of This gives rise toa gravitational unbalance which renders the heavy end of the needle responsive to acceleration forces. Secondly, the magnetic element must be made pendulous if it is to give a useful indication. This is due to the fact that the magnetic element must give an indication of direction in azimuth and unless the magnetic element is stabilized in a horizontal plane so that its movements take place in substantially said plane, no useful indication would be given by the magnets. The reason for this is that the magnets, if nonp'endulous and therefore unstabilized in a horizontal plane, are free to rotate about the axis through the N-S poles. Making the magnetic element pendulous, however, renders this element responsive to every change in speed and direction, giving rise to large errors, sometimes in Some magnet c compasses, in practice, are made excessively pendulous in order to avoid the necessity of applying counterweights to counteract the dip, but such compasses are subject to the same errors due to change of speed or direction.

The problem consists in introducing in a magnetic compass the necessary pendulous factor so that the said compass may indicate direction in azimuth, and at the same time free the magnetic compass from errors of such magnitude as will render it useless upon-changes of speed or tuming of the craft upon which it is mounted.

Applicant accomplishes this desirable result by so mounting a pendulous element on the magnetic element that the said magnetic and pendulous elements are rigidly connected and act like a rigid pendulum within a predetermined small angle of displacement from vertical or until a predetermined acceleration force. acts thereon, whereby stabilization of the magnetic elementin azimuth is obtained, but causing the said rigid connection to be broken when said predetermined angle or acceleration force is exceeded whereby further displacement of the pendulous element does not apply an increasing disturbing force on the magnetic element.

The invention has for a further object the provision of an instrument capable of functioning as an attitude of flight indicator independently of direction indicationand which will not be substantially affected by acceleration forces.

Further objects and advantagesof this invention will become apparent in the following detailed description:

In the accompanying drawings:

Fig. 1 is a vertical section through a compass embodying one form of the invention.

Fig. 2 is a plan view of the Fig. 1 device with parts of the casing broken away to disclose the underlying mechanism.

Fig. 3 is a vertical section through the sensitive element comprising the magnetic element and the pendulous element.

Fig. 4 is a detail of the pivot post whereby the sensitive element is mounted.

Fig. 4-A is a view similar to Fig. 4 on a smaller scale, showing the parts in a diflerent angular relationship.

Fig. 5 is a view showing the principle involved in the construction of the cup for supporting the pendulous element.

Fig. 6 is a view similar to Fig. 5 showing amodifled form of cup for supporting the pendulous element.

Figs. '7 to 11, inclusive, are diagrams illustrating the underlying theory of the invention.

Figs. 12 and 12A are end views largely diagrammatic, of a portion of the magnetic element of Fig. 2, illustrating the function of the stop mechanism.

Fig. 13 is a view similar to Fig. 1 showing another form of the invention.

Fig. 14 is a front view of the sensitive element within the Fig. 13 device.

Referring to the drawings, it will be seen that I have disclosed a type of compass which is free of the defects present in all previous forms of magnetic compass and enables the pilot to fly in any direction and to turn, always directing himself solely by the compass and not relying upon other instruments.

To accomplish the above, I have provided a sensitive element. which comprises essentially three parts, first, a magnetic or meridian-seeking element, second, a pendulous element, and, third, means for supporting the pendulous element on the magnetic element. The magnetic element consists of a magnet system and a floating chamber or card element upon which the system is mounted. The magnet system may consist of a pair of magnets l0, l fixedly enclosed within closed chambers l2 and I3, re-

spectively. The said chambers are fixedly connected to the shaft so that said shaft is constrained to move with said magnets as the latter change their dip or inclination. The said shaft I is mounted for rotation about its axis on pivots l5, l6, journaled in bearings I1 and I8, respectively, said bearings being carried by a cylindrical float chamber 20. The said float chamber consists of an outer cylindrical wall 2|, an inner cylindrical wall 22, a top 23 and a bottom 24. The inner cylindrical wall 22 forms a hollow center for the float chamber within which is adapted to be mounted the supporting mecha- Fig. 2) in which the magnet float chambers |2 and I3 operate.

The magnetic element, as thus far described, is a buoyant member which is designed to be placed in a container C filled with fluid. Said container may take the form of a sphere consisting of a plurality of parts 27, 28, the part 21 communicating with an expansion chamber 29 to take up variations in the volume of the liquid due to temperature changes, and the part 28 having a window 39 in the front thereof through which the compass readings may be taken. .The magnetic element being buoyant, this fact is taken advantage of to support said element against a downwardly extending pivotal bearing 30 at the lower end of a pivot post P fixed in the casing C. For this purpose, the magnetic element is provided with a pivot 3| bearing against a cup 32 formed in pivotal bearing 30. The pivot 3| may take the form of aspherical jewel or metallic member, such as platinum-iridium al- 10y, operating in cup bearing 32 which is of slightly greaterdiameter than pivot 3| so that the pivot 3| is internally tangential to cup 32 at substantially a point 50. The said pivot 3| may be formed on the upper end of a post 33 screwthreaded at its lower end into a plate 34 having flanged portion 35 which is supported for movement in a horizontal plane within circumferential groove 36 formed by upper and lower rings 31 and 38 fixedly supported on the inner wall 22 of the float chamber 20. The buoyancy of the float chamber 20 and the mechanism supported thereby is such that thesaid chamber 20 and all the mechanism supported thereby will be held upwardly in engagement with cup 32 of pivot post P.

The said pivotal connection between pivot 3| and bearing 30 may be enclosed within a retaining casing freely mounted on the bearing cup and comprising upper and lower members 55, 56, surrounding the bearing member 30 and the bearing post 3|, respectively, and spaced by space '32 and counteract any tendency for the sensitive element to leave its bearing in response to jars, vibration,-and the like. The said retaining casing is freely mounted on the said bearing members so as not to interfere with the relative angular movements of post 33 with respecttc post P.

From the above description it will be apparent that we are enabled to obtain a maximum relative angular movement of the sensitive element with respect to pivot post P without disengagement of the parts. This is due to the fact that the pivot 3| is as nearly a complete sphere as is possible, only a minimum portion thereof being utilized for connection to the post 33. The inner surface 32 of cup 30 fits the bearing 3| rather closely and extends down less than half the diameter of the sphere, so that a maximum movement of the sensitive element with respect to the said pivot post P of almost 60 to either side is made possible (see Figure 4A). This is in contra-distinction to the standard form of magnetic compass where a needle point engages in a shallow cup and after a maximum of 20 displacement the needle slides in the cup and may become completely disconnected. Any tendency of pivot 3| to be dislodged frombearing cup 30 is counteracted by'the retaining casing which, however, because of its loose mounting, does not interfere with the relative angular movements of the sensitive element with respect to the pivot post F. It will now beseen that by the construction thus far described, the magnets l3, I0 are free to assume their natural angle of dip by reason of the fact that they are carried by axle II in fixed relation to said axle which-is free to rotate about its axis in bearings I8. It is also apparent that the sensitive element as a whole may rotate around a vertical axis through the pivot and bearing. Under these circumstances the magnets have two stable positions with respect to the sensitive element. Ordinarily, the north end of each magnet operates in a vertical semi-circle indicated at A in Figure 12, as the compass travels from the north to the south pole, and the marking on the surface 2| can be read. through window 35 in conjunction with lubber line I42. Thus, if the craft is traveling north, the letter N on the card element will be seen through window 39. However, if the sensitive element should for some reason become turned through then the semi-circle A takes up the position shown in Figure 12A. This is an unstable position for the magnets, since the north end is pointing south, and the north end of each magnet. therefore tends to travel by the shortest route, indicated by the arrow, to the dotted line position where the north end again points north. That is to say, the north end of the magnet tends to swing around the axis |5- |5 through the vertical plane into the semi-circle B. Since the north end of each magnet is now pointing north again, this is a stable position, even though the sensitive element and the compass markings are turned through 180". In other words, if the craft conof the magnets.

tinues to travel north, the south markings on wall 2| will be seen through window 39. The readings are thus 180 in error. To prevent such error from arising, we provide means for confining the north end of the magnet to semi-circle A and the south end to semi-circle B by preventing'passage of the ends of the magnets through the vertical line indicated at V which is in reality a vertical plane passed through the dip axis ll of the magnets. In other words, the north end cannot, in Fig. 12-A, travel the course indicated by the arrow into the semi-circle B. For this 1 purpose we provide on the float chamber 20 within the cavities 25, 26 in which-the magnets operate, a stop H in each of said cavities, which stop is so positioned that it will engage one of a set of lugs carried by each float chamber l2 and I3 in which the magnets are housed. The

stops and lugs are so positioned that the stops will engage one or the other of the lugs of each set before the end of the magnet can'pass through the said vertical plane through the pivotal axis Therefore, should the sensitive element become turned through 180, there is no way by which the north end of the magnetscan again indicat north except by turning through 180 in azimuth and necessarily carrying the sensitive element through 180 in azimuth to its original settling position.

Thus the sensitive element is freed of the first of the two objectionable features present in ordinary magnetic compasses, namely, the gravitational instability which is caused by placing weights on one arm of the pivoted magnetin order to bring said magnet into the horizontal position, and therefore there is avoided the error which present magnetic compasses are subject to by reason of the action of acceleration forces on the offset weight.

As stated hereinbeforc, it is necessary that any magnetic meridian seeking element be stabilized in the horizontal plane, and this is effected by rendering the samependulous. The pendulous element here employed may comprise a ball B which is supported on the magnetic element by a circular cup 40 which is fixed in position on inner wall 22 so as to close the lower end of the hollow The said cup 40 may be fixed in position in said hollow center formed in float chamber 20.

interior by means of ring al. The ball B operates on .the surface 60 of cup .40 and imparts the necessary pendulosity to the entire sensitive element so that it is stabilized in a horizontal plane.

However, the weight of the sensitive element as,

a whole including the pendulous element B is still such that it is less than the weight of the displaced fiuid, and therefore the sensitive element will be held against the cup 32 at the end of pivot post P. However, the buoyancy is such that the sensitive element will touch the point of tangency 50 in the said cup 32 with a pressure which is the minimum necessary to ensure engagement so that pivot friction isreduced to a minimum.

It is essential for the proper operation of this type compass that the center of gravity and center of buoyancy of the magnetic element and cup 40 shall coincide. For this purpose, the ad-' Justing screw l8 may be utilized, the said screw supporting the journal portion 16 carried by shaft l'l. This, however, shifts the center of gravity only in one of three essential directions, and for the other direction in the horizontal plane small masses of metal may be placed uponthe cylindr al wall 2| to efiect the necessary balance" and if desired such masses of metal maybe used in the direction of axis II also. In the third direction, the center of gravity is adjusted by means of a ring 63 which may be operated vertically by a plurality of adjusting screws 44 extending through said ring and threaded into grooves in a ring 65 fixed to the inner wall 22. When the float chamber 20 with all of the elements supported thereby, except the pendulous element B, and with the magnets demagnetized, is suspended in the liquid and it remains in whatever position it is placed, then the center of gravity and the center of buoyancy coincide. The said adjustments and placing of weights-are continued until such coincidence of the center of gravity. and center ofbuoyancy is effected.

It is next necessary to cause the center of gravity and center of buoyancy of the magnetic element and cup 40 to coincide substantially with the center of pivot 3i. For this purpose, the sensitive element may be adjusted in two ways in the horizontal plane by means of pairs of screws justment in a horizontal plane along two axes at right angles to each other may be effected to cause substantial coincidence of the center of pivot 3| with the center of gravity and center of buoyancy in the vertical axis through post 33. This is accomplished by loosening up on one screw of. apair and tightening theother screw of said pair. However, the said center of gravity and center of buoyancy may be above or below the center of pivot 3 l, and in order to make them substantially coincide a vertical adjustment may be effected by screwing post 33 into or out of member M. For this purpose, the saidpost may be provided with a toothed wheel 52 on a screw threaded into plate 3t,-whereby turning of wheel 52 effects raising or lowering of the float chamber 20 until the said center of gravity and center of buoyancy substantially coincide with the center of pivot 3|. Proof of' coincidence is obtained when the sensitive element (exclusive of 'bail B but including support 40, and with the magnets demagnetized,) retains any position in which it may be placed on cup 30.

When such coincidence has been effected, the pendulous element B is placed in cup 40. The

exact degree of pendulosity is thus known, and in the present case such pendulosity is of the very of the sensitive element so that it may be maintained in a horizontal plane.

As thus far described, the sensitive element, consisting of the magnetic meridian-seeking element, the pendulouselement, and the support therefor, constitutes a very slightly pendulous magnetic compass. The fact that the pendulosity of the sensitive element is very slight, being only suflicient to enable it to overcome pivot friction and maintain itself in a horizontal plane, will prevent any great errors from being introduced in response to acceleration forces, the said term acceleration forces" being used here in its broadest sense to include change of speed in a straight line or turning in any direction. Furthermore, in addition to reducing the error due to pendulosity 'by making the pendulous factor the minimum essential quantity, such error in response to acremains substantially unresponsive to all disturbing forcesaround said horizontal axes except those of long period.

The compass as thus far described has made provision for three important means foreliminating errors due to acceleration. These are as follows:

l.-The magnets are free to take up their natural angle of d;p and therefore no counterweighting is necessary and therefore no acceleration errors due to this gravitational unbalance are possible.

2. The pendulosity of the sensitive element has been reduced to such a minimum that there is only sufllcient pendulosity to overcome the friction in the bearings, and therefore the errors due to such pendulosity are held to a minimum.

3. The inertia of the sensitive element has been made so great that the said element will not be substantially affected by short period disturbing forces.

In spite of all the above steps to reduce the errors caused by acceleration forces, the fact remains that the sensitive element, being pendulous, although only slightly so, would normally respond to large acceleration forces, were it not for the fact that we make provision for eliminating the possibility of such errors. For this purpose, we provide means for uncoupling the pendulous element from the magnetic element when the said pendulous element has been displaced from the vertical beyond a predetermined,

relatively small, critical angle, or whenever the d sturbing forces such as acceleration forces, acting upon the pendulum exceed a predetermined force. For this urpose, the inner surface 60 of cup 40 is formed as shown in Figure 5, not as a true sphere but as a surface of revolution formed by rotating an arc of a circle of radius R around an axis OO. The point coincides with the center of pivot 3|. As a result, there is formed in the extreme lowest point of the cup a notch or depression N in which the ball B will lodge until a force of suflicient magnitude dislodges it or until the cup 40 is displaced a suflicient angle to cause the said ball to roll'out of the notch on to the smooth surface 60. As long as the ball B remains in the notch N the entire sensitive element moves as a unit and operates as a simple pendulum obeying the law of the simple pendulum as shown in Figure 8. Thus, if the angle of displacement is 0, the length of the pendulum l, and the weight of the ball indiicated as my, then the restoring torque equals mgl sin 0, and the greater the angle of displacement the greater is the restoring torque, so that if plotted as a graph, as shown in Figure '7, a sine curve results. However, as soon as the acceleration force increases sufliciently to cause the ball to leave the notch, the ball and cup cease to act as a unit and the restoring torque no longer follows the famliar mgl sin 0 curve. Referring to Figure 9, we see that the disturbing torque is cos 0 and as a result the disturbing torque=mga. From this we see that once the ball leaves notch N it applies a constant torque to the cup, and, hence, to the magnetic element, equal to mm: which is independent of 0 and therefore independent of the size of the acceleration force.

Xa cos 0 Plotting this result graphically in Figure 'l, we

find that a horizontal line results, indicating constancy of torque. The above result is not surprising because we know that a ball operating on a cup which is perfectly spherical exercises no torque thereon because the resultant force always acts through the center 0 no matter how far the ball is displaced (see Figure 10) and therefore the ball exerts no torque on the cup. The reason that in Figures 7 and 9 a constant torque prevails after the ball leaves the notch N in the cup, rather than zero torque as in the Figure 10 form, is that O" (the center of the arc) in Figure 5 isdisplaced from O, O( (the axis of revolution) the distance equal to a.

When the cup is displaced angularly from vertical without centrifugal force being present, the ball will leave the notch N after a few degrees, as shown in Figure 11. In this case, the restoring torque is mgXa cos 9=mga cos 9. This is a maximum when 0 is zero and is zero when 0 is in accordance with the sine curve (see Figure 7) until the ball leaves the notch and thereafter the torque falls off, as the angle of displacement from vertical increases, in accordance with the formula mga cos 0. I

The distance a which center 0" is displaced from O is such that mga (the maximum torque which the ball will transmit to the magnetic element) is just sufficient to overcome pivot friction. In practice, this is found to be such that when the cup is displaced from the vertical about 5 the ball will leave the notch N. This means that within 5 from vertical the ball and cup act like a rigid pendulum to provide the necessary pendulosity on the compass to maintain the compass in a horizontal plane so that direction in azimuth may be indicated, but that beyond this angle (or when a centrifugal force equal to or greater than mga is applied) the ball rolls freely on the cup and the torque which it applies to the cup will not be further increased. The force mg'a which the ball may apply to the cup,

and therefore to the magnetic element is so slight (being only sumcient to overcome pivot friction) that large acceleration forces will not introduce any substantial error.

Instead of the notch N formed by the surface of revolution, a detent N, as shown in Figure 6, may be employed. The groove N and the steepness of the surface of revolution are such that ball B will be dislodged from said groove by the same acceleration force which will cause it to leave notch N.

In this manner, applicant has solved the problem of providing the necessary pendulosity to render a magnetic compass operative without laying the-compass open to the substantial errors due to acceleration forces acting upon the pendulous factor, which errors have in the past been of such magnitude as to render the compass ineffective for the purpose of guiding the flight of a craft.

The usual airplane compass markings are placed onthe outer surface of wall 2| of the card element and are read in conjunction with lubber line 2 supported by the casing back of window 39.-

Another embodiment of the invention which operates upon the same principle and yields the same desirable results is illustrated in Figures 13 and 14.

Within the same casing C having the same pivot post P, there is mounted a buoyant lensi- The restoring torque, therefore, operates tive element consisting of the sametwo funda mentals, namely, a magnetic,meridian-Seeking element and a pendulous element. The form disclosed in Figures 13 and 14 differs from the form already described only in the respect that the buoyant members are of somewhat different construction and the pendulous element differs from the type of pendulous element hereinbefore described.

The magnetic element'comprises sets of magnets III), III! and III, III which are mounted between ring H2 and a float chamber H3. Said mounting may be accomplished by means of pivot pins II pivoted at H5 and Il6 to the ring and to the float chamber, respectively. The said float chamber II3 may comprise one or more float type compartments Ill. The center of the float is hollow and provides a support H8 for the cup I I9 within which the jewel support is fixed. The magnets IIIl, III! and III, III' may also be enclosed within float chambers I20, said shafts II extending through the said float chambers I211. The entire magneticmeridian-seeking element is of such dimensions and values that it is designed to float in the liquid which fills the thus the full centralizing effect of the pendulum is transmitted to the sensitive -element.- When this small critical angle is exceeded, however,

then certain of the flexible connections become limp and the remaining connection, which is the only taut one, pivots around its point of connection I 40 or I 40', etc. so that the force is no longer transmitted through the lever I3I, I3I etc. but through the short arm from the pivot J perpendicular to the taut flexible connection. In

' other words, as soon as the critical angle is exof Figure 7. It will thus be apparent thatthe casing C, so that jewel J will rest firmly against the end of pivot post P.

It will be observed as in the Figure 1 form of the invention that the magnets are free to take up-their natural angle of clip by pivoting around pivots H5 and IIS. Thus there is no need for weighting one end of each magnet and hence the acceleration'which. arose by reason of the weighting of one end of the compass as heretofore employed, is avoided. The entire magnetic meridian-seeking element as heretofore described is non-pendulous and is adjusted so that the center of buoyancy and the center of gravity coincide at the pivotal point J.

The pendulous factor in this case consists of a pendulous mass I attached to the magnetic element by means of connections I3I, I 3|, I3I". The weight of the mass I30 is, however, the absolute minimum amount which, acting through the members ISI, I3I', I3I' will overcome the friction in the bearings and stabilize the sensitive element in the horizontal plane. This amount is exceedingly small. This means that the effect of acceleration forces upon the said pendulous I mass will be very slight.

Not only is this pendulous mass which is necessary to stabilize the magnetic element in ,a horizontal plane of exceedingly small dimensions, but provision is made whereby the maximum gravity and acceleration forces transmitted by the pendulous element to the magnetic element are but a small fraction of the gravity and acceleration forces which may act on the pendulum. For this purpose, we connect the pendulous mass I 30 to the magnetic element, not by a rigid connection, but by a yielding connection. Thus, in the form shown in Figure 13, the pendulous mass is connected to the magnetic element by a plurality of threads I3I, I3I', etc. which are connected to the magnetic element at points such as I60, I80, etc.

With the above construction it is found that the set of threads ISI, I3I', etc. act like a rigid member within a small angular displacement either sideof the settling position, that is to say, for two or three degrees either side of the vertical the said threads act like-a rigid unit to give the maximum centralizing effort. Within this range, forces acting upon the pendulum follow the line mgl sine portion of the curve of Figure 7, and

, of the invention.

principle embodied in the Figure 13 form of the invention is the same as in the Figure 1 form The rigidity of the threads within the critical range corresponds to the lodgment of the pendulous ball Bin the notch N of the Figure 1 form, and the pivoting of the threads around their points of connection Mil, etc. co'nstitutes the loosely coupled connection corresponding to the movement of ball B outof notch N. p

In the Figure 13 form of the invention the scale markings arecarried on the outer surface of ring II 2 as shown in Figure 13, while in the Figure 1 form of the invention the scale markings are carried by the outer wall 2i of the float chamber 20. A lubber line It! may be fixed in position back of front window 39.

In both forms of the invention described hereinbefore it will be apparent that the float is not appreciably affected by acceleration forces. Therefore, each of these forms can be used as an attitude-of-flight indicator in the form of an artificial horizon and inclinometer at the same time as these instruments indicate direction. For this purpose, horizontal lubber line I63 coincides with the horizontal mark on the float.

If direction indication is not desired, the'magnets may be replaced by unmagnetized masses -of the same magnitude, and the device will function as an attitude-of-flight indicator independently of direction indication. This is apparent from the fact that the entire float element is slightly pendulous. but because of the fact that the pendulum is loosely coupled to the floating mass in response to acceleration forces in excess of the predetermined magnitude, the device is substantially unaffected by acceleration forces and therefore is not subject to the errors which haveheretofore characterized pendulous artificialhorizons and inclinometers.

In accordance with the provisions of the patent statutes, I have'herein described the principle I and operation of my invention, together with the apparatus which I now consider to represent the best, embodiment thereof, but I desire to have it understood that the apparatus shown is only illustrative and that the invention can be carried out by other equivalent means. Also, while it is designed to use the various features and elements in the combination and relations described, some of these may be altered and others omitted without interfering with the more general results outlined, and the invention extends to such use.

Having described my invention, what I claim and-desire to secure by Letters Patent is:

1. In a device of the character described adapted to be mounted on an aircraft, a sensitive element, means for supporting said sensitive element for universal pivotal movement, said sensitive element including a magnetic directionseeking element, a pendulous element, and means for supporting said pendulous element on said magnetic element, said last named means including a cup carried by said magnetic element and having an inner surface of revolution, said cup supporting said pendulous element for movement upon said inner surface, said inner surface having detaining means for normally retaining said pendulous element in place up to a predetermined small angle of tilt.

2. In a device of the character described, a casing having fluid therein, a bearing carried by said casing, a buoyant sensitive member supported in said fluid against said bearing for universal pivotal movement, said member comprising a magnetic element, a pendulous element,

' and means for supporting said pendulous element on said magnetic element, said magnetic element including a float chamber, a shaft rotatably supported in said chamber and having its axis lying in a vertical plane through said bearing, and a plurality of magnets fixed to said shaft on opposite sides of said bearing.

=3. In a device of the character described, a casing having fluid therein, a bearing carried by said casing. a buoyant sensitive member supported in said fluid against said bearing for universal pivotal movement, said member comprising a magnetic direction-seeking element, a pendulous element, and means for supporting said pendulous element on said magnetic element, said magnetic element including a float chamber, a straight horizontal shaft rotatably supported in said chamber below said bearing and having its axis lying in a vertical plane through said bearing, and a plurality of magnets fixed to said shaft on opposite sides of said bearing.

4. In a device of the character described, a casing having fluid therein, a bearing carried by and within said casing, a buoyant sensitive member supported in said fluid against said bearing for universal pivotal movement, said member including a magnetic direction-seeking element and a pendulous element, said magnetic element including a float chamber, a shaft rotatably supported in said chamber, a plurality of magnets fixed to said shaft on opposite sides of said bearing, means whereby said member exclusive of said pendulous element may be gravitationally and bydrostatically balanced to cause its center of gravity and center of buoyancy to coincide, and means whereby said point of coincidence may be made substantially coincident with the point of support of said member on said bearing.

5. A direction indicator including, in combination with a container having a smoothly spherical interior surface, a fluid substantially filling said container, a freely rotatable member hydrostatically supported in said fluid, and a magnet pivoted upon said member to rotate with respect to said member in a normally vertical plane and automatically to align the magnet poles with the dip of the earths magnetic fleld and to orient said member to a desired position in azimuth.

6. A device for stabilizing direction indicators with respect to gravity while reducing the disturbing efiectof acceleration forces thereupon, including a relatively non-pendulous member rotatable about any axis normal to the direction of stabilization, a small unattached mass for supplying the principal effective pendulosity of said member, and a cam having a concave spheroidal surface, forming a part of said member, for supporting said mass and converting radial pressure due to action of the stabilizing force and other acceleration forces upon said mass into torque upon said member in a desired proportion to the angle separating the direction of said other forces from the normal stabilization axis of said member, said mass being freely movable over said cam surface, and said cam surface having one portion most remote from its spheroidal center into which said mass falls in the absence of said other acceleration forces.

'7. In a magnetic compass, a casing, a sensitive element mounted'within said casing for universal movement with respect thereto, comprising a support and a direction-seeking magnet pivoted on said support for movement about a transverse normally horizontal axis, said support being formed with a cam surface of revolution generated by rotating an end portion of an arc of a conic section about the chord thereof, said chord being normally vertical, the full arc of said portion extending less than and a free mass supported on said surface and actin thereon to normally urge said support into a position to main- 

